I have some cucumber plants (sorry, not sure which type), with a couple in a large, long pot, and one other in a small pot by itself. The plant in the small pot has a reasonably-sized cucumber, about 5 inches long, while the other plants are struggling to grow smaller ones. I live in an apartment in Southern Canada, on a high floor, and the plants are on a balcony facing west. They get a lot of sunlight in the afternoons and evenings.
The plants started off well, with some white around the edges from an early frost, but they grew quickly through the late Spring and early Summer. However, within the last several weeks, the leaves have undergone a drastic change, turning yellow and white, drying out, and then dying. All the leaves, especially the older ones, and now the newer ones, are shriveling up and dying. At this point, all the leaves on the solo plant are like this, and the other plants' leaves are only about 75% green. There's also a yellowish powder on the old leaves when I rub them, but that could simply be part of their decomposition.
Is there anything I can do for my cucumber plants? Like I said, they get an abundance of sunlight, and there's been little rain (I water them once in the morning). I occasionally give them a general fertilizer (20/20/20), which helped my tomato plants bounce back but hasn't done anything for my cucumbers.
I briefly had spider mites on one of the plants, but several days of spraying insecticide soap seemed to kill them. The photo below was taken around the same time.